Rat renal allografts survive, free from rejection (enhanced), if the recipient is treated with a combination of donor strain antigen and anti-donor antibody at or shortly before the time of transplantation. Long-term stable recipients retain a variety of immunologic reactivities against donor antigens but fail to reject the kidney. Our studies are directed at clarifying the nature of the residual immunologic reactivity and defining the nature of the balance that allows continued acceptance of the graft.